DIGESTION IN THE STOMACH. 69 



trol of a nerve center which transmits influences that tend to re- 

 lax the sphincter when the afferent fibers running to it from the 

 stomach side are excited by acid, but which cause it still more 

 powerfully to contract when the acid acts on afferent fibers hav- 

 ing their terminations in the duodenum. When both afferent 

 paths are simultaneously stimulated, the duodenal predominates 

 over the gastric, so that the sphincter remains closed until the 

 acidity of the chyme in the duodenum has all been neutralized, 

 and this seems to be true however faint the acidity may be on 

 the duodenal side and however strong on the stomach side. The 

 reflex arc is situated in the walls of the pyloric region and duo- 

 denum, for it operates after complete isolation from the central 

 nervous system. It is a function of the plexus found present in 

 the walls the myenteric plexus. 



Rate of Discharge of Food from the Stomach. The acidity 

 of the gastric contents, as we have just seen, must attain a cer- 

 tain degree before it becomes an adequate stimulus for the open- 

 ing of the pyloric sphincter, and consequently the rate at which 

 the different food stuffs leave the stomach is to a large extent 

 proportional to their power of combination with the acid. Pro- 

 teins, combine with large amounts of acid, so that their initial 

 discharge is delayed and their subsequent passage slow. Car- 

 bohydrates absorb but little acid, so that they begin to leave early 

 and the stomach is soon emptied of them. The passage of fats is 

 peculiar; when taken alone, which, however, is scarcely ever the 

 case, they seem to bring about a partial relaxation of the pyloric' 

 sphincter, so that bile and pancreatic juice regurgitate into the 

 stomach and some fat may pass out, but the subsequent dis- 

 charge into the intestines is very slow, so slow indeed that each 

 discharged portion seems to become completely absorbed before 

 any further discharge occurs. When fats are mixed with other 

 foods, they materially delay the discharge. These effects are 

 no doubt due in part to the inhibitory influence which fats have 

 on gastric secretion ; and in part to the liberation of fatty acid in 

 the duodenum by the action of pancreatic lipase. This fatty acid 

 seems to be liberated more quickly than it becomes neutralized 

 by alkali. 



